Update to FHS v3.0

Notable changes that this causes:

* A GNU-style /usr/libexec is now allowed.

Notable non-changes:

* /usr/games, /usr/share/games and /usr/lib/games were optional in
  FHS 2.3, and are still optional. It is up to us whether we want
  to keep using those directories.

Drop our special exception for /run, and replace it with a requirement
that /var/run and /var/lock are symlinks to /run and /run/lock
respectively. The FHS does not mandate that these directories are
symlinked or bind-mounted (although I think it should) and some
misguided distributions make /run and /var/run non-equivalent, but they
have always been equivalent on Debian.

Drop our special exception for /sys, which is now standardized in the
FHS.

Relax the requirement to create /usr/local/share/color to a
recommendation to avoid making 6 packages immediately noncompliant.

Add a special exception for /usr/bin/mh/ for now, restoring the FHS
2.3 situation (dropping this might be a good idea, but should be
disussed with the nmh and mailutils-mh maintainers if desired).

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #787816